A helicopter is mostly a machine for dealing with the consequences of its own engine. Put a motor in the fuselage and you need a gearbox to get the power up to the rotor, a shaft to carry it, and a tail rotor to stop the fuselage spinning the opposite way to the blades. Most of the weight, most of the complexity and a good deal of the danger live in that chain.

Hiller Aircraft’s answer, in 1948, was to delete all of it. Put the engines at the blade tips and let them pull the rotor round. No gearbox. No driveshaft. No tail rotor. The result was the Hornet, and it flew.

Why a tip-driven rotor needs no tail rotor

A conventional helicopter’s tail rotor exists for one reason: Newton. The engine twists the rotor one way, so it twists the airframe the other, and something has to push back. That something is a whole second rotor, its own driveshaft, and a control system, all of it dedicated to cancelling a side effect.

Drive the rotor from the tips instead and the torque never reaches the fuselage. The engines are out at the ends of the blades pulling them around, so there is nothing for the airframe to react against.

The weight saving is not marginal. The Hornet came out at 544 lb empty and could carry a payload of 536 lb, which is very nearly its own empty weight again. Almost no helicopter manages that, before or since, and it is a direct consequence of having no transmission to carry.

Why ramjets were the wrong engine

A ramjet has no moving parts. It uses its own forward speed to compress incoming air, burns fuel in it and pushes the result out the back. That simplicity is exactly why it appealed to Hiller, and it is also why the Hornet was doomed.

Ramjets only become efficient when the air is arriving fast enough to be compressed properly, which in practice means supersonic. A helicopter rotor tip does not go supersonic; if it did, the aircraft would shake itself apart. So the Hornet’s ramjets spent their entire working lives operating in the regime where they are least efficient.

The numbers tell the story without further comment. Twenty-five minutes of endurance. Twenty-eight miles of range in still air. An aircraft that can lift its own weight in payload and then take it almost nowhere.

There were other problems, and they were the sort a military customer notices. The noise was extraordinary, because the sound source was whirling around at the end of a 23-foot rotor. At night the ramjets glowed, making the aircraft visible from a considerable distance. And if the engines quit, autorotation was difficult, because the drag of the ramjet nacelles out at the blade tips worked directly against the rotor keeping its speed up. On a helicopter, a compromised autorotation is not a minor complaint.

Sally Rand, and the first helicopter gunship

Two YH-32s were converted into armed YH-32As for trials, and one of them acquired the name Sally Rand, after the fan dancer. The Hiller Museum identifies the YH-32A as the first helicopter gunship, which makes this awkward little machine an ancestor of the Cobra and the Apache.

It is worth keeping the claim in proportion. This was live-fire testing of a small experimental helicopter with weapons attached, not an operational attack aircraft. But the idea that you could arm a helicopter and use it to shoot at things on the ground had to start somewhere, and it started here.

Warner Pathé newsreel from 1951 showing the Hornet flying, with the ramjets lit at the blade tips.

What it was actually worth

Eighteen aircraft were built and none entered service. The YH-32 beat the rival XH-26 Jet Jeep, and then the whole idea was judged obsolete and cancelled.

That reads like failure, and commercially it was. But tip-drive did not die with the Hornet. The Dutch NHI Kolibrie used the same principle, and the idea keeps resurfacing whenever somebody wants maximum lift for minimum mechanical complexity. The physics that killed the Hornet was never the tip-drive concept. It was the specific choice of a ramjet, an engine that wants to go supersonic, bolted to a rotor blade that must never be allowed to.

Put a different engine out there and the arithmetic changes. Hiller simply picked the only jet engine simple enough to survive being thrown around a circle at the end of a rotor blade, and paid for that simplicity in fuel.

Vertical Flight Society footage of live fire testing with the armed YH-32A, the Sally Rand.

A modern look at the Hornet and how its ramjet rotor actually worked.

Sources: Wikipedia’s Hiller YH-32 Hornet article; Wings of History Air Museum on the Hiller 8RJ2B ramjet; Hiller Aviation Museum; Vertical Flight Society.